The present invention relates to a blind fastener wherein a main sleeve and expander sleeve are assembled onto a pin shank of a pin member having an enlarged pin head with the expander sleeve at the blind end in engagement with the pin head such that as a relative axial force is applied between the pin and the sleeves the expander sleeve is moved into the adjacent end of the main sleeve to expand that end radially outwardly to form a first tulip shaped blind head and with the pin head then moving into the expander sleeve to form a second tulip shaped blind head.
In securing workpieces together with a blind fastener it is desirable to be able to attain a high final clamp load on the workpieces. At the same time it is also desirable that the installed fastener have the capability of withstanding high shear loads. The magnitude of clamp load is established by the relative axial force applied between the pin and a sleeve structure essentially when the blind head is formed and engages the rear surface of the inner workpiece. Typically, as this is occurring, the pin moves axially relative to the sleeve structure until a breakneck groove reaches an enlarged sleeve head at the outer end of the outer workpiece where a front lock is formed and the pin is fractured at the breakneck groove to result in a flush final structure. The extra load to provide pin break, however, does not necessarily increase the clamp load.
The form and strength of the blind head and lock structure of the installed fastener securing the final magnitude of clamp load can be a significant factor in determining the shear load capability of the installed fastener and fastened joint. In many applications the shear load capability of a fastened joint is of prime importance. Shear loads are present on the installed fastener through the fastened joint by loads applied to the workpieces in opposite radial directions which are transverse to the axis of the fastener. This form of loading is frequently referred to as lap shear and, of course, is in contrast to pure tensile loads which are applied to the workpieces in opposite axial directions along the axis of the installed fastener. When such lap shear loads are applied to a fastener, however, there routinely is a component of resultant tensile load applied to the fastener as well. It is believed that this is caused to some extent by the fact that since the workpieces are axially offset the shear loads as applied to the workpieces in opposite transverse directions, will result in some degree of bending or twisting which will cause an uneven axial load to be applied to the installed fastener. The pin and sleeve, however, are routinely, axially retained together to resist tensile loads. Such retention is frequently provided by the enlarged sleeve head, the lock formed by the locking ring or collar at the outer or open end and by the blind head at the inner or blind end. However, the uneven tensile loads or forces resulting from the lap shear loads can cause some axial displacement or deformation of the blind head structure relative to the front lock and/or the enlarged sleeve head. This could result in a reduction in the clamp load whereby the workpieces could now rotate or otherwise move relative to each other resulting in an ultimate deterioration of the fastened joint.
In the present invention, this problem is alleviated by forming a high strength blind head structure having two, interengaging blind heads of a tulip configuration and by overengagement of the pin head by the blind head structure. At the same time the clamp load attainable can be of a high magnitude. Here the first blind head is formed in a main sleeve by an expander sleeve. The expander sleeve is mounted in engagement with the pin head and is moved into the blind side end of the main sleeve in response to a relative axial force applied between the pin and sleeves to form the first blind head of a tulip shape to engage the inner workpiece surface and to thereby initiate clamping of the workpieces. Now as the relative axial force is increased the expander sleeve is moved fully into the main sleeve to further increase the clamp load while substantially increasing the strength of the blind head. In this regard the pin head and expander sleeve are provided with interengaging flat, non-tapered surfaces whereby a high strength blind head is formed and a high clamp load can be attained before the pin head moves into the expander sleeve. Now with the expander sleeve fully seated within the main sleeve, as the relative axial force continues to increase, the pin head is moved into the inner end of the expander sleeve to radially expand the expander sleeve to form the second blind head also of a tulip shape. At the same time this acts to further strengthen the blind head and to secure the first tulip shaped blind head on the main sleeve with the second tulip shaped blind head on the expander sleeve. This results in further securing the fastener members at the blind head side. While this is occurring the pin shank is moved gradually, with the clamp load maintained, to a desired final position where a front lock is engaged to lock the pin member and main sleeve together and at which position the breakneck groove is essentially in line with the outer end of the main sleeve. Now as the relative axial force is increased further the breakneck groove will be fractured completing the installation.
As this happens the expander sleeve because of its resilient strength in the area of the second tulip shaped blind head will exert a radially inward gripping force on the pin head. At the same time it will move radially into engagement with and over the pin head to help retain or lock the expander sleeve and pin member from moving relatively to each other which could result in clamp load being reduced or lost. In addition the leading end of the expander sleeve which is engaged by the material of the tuliped main sleeve will be urged radially inwardly into engagement with the pin shank to assist in retaining the expander sleeve to the pin member and main sleeve. Thus these areas of retention or locking engagement at the blind side are in addition to the front lock securing the pin member and main sleeve together to thus further secure the joint. These retention features plus the high strength of the blind head and the high magnitude of clamp load attainable increase the resistance of the fastener components from loosening from the tensile forces resulting from lap shear loads.
At the same time the expander sleeve and main sleeve are constructed such that the distance the pin head will move into the expander sleeve will vary depending upon the grip range of the fastener, i.e. to accommodate variations in total thickness of the workpieces being joined. Such variation of inward movement, however, will occur only after the blind head structure has been fully formed and the above noted retention structures have been formed thereby having little effect on the magnitude of final clamp load attainable and the strength of the structure retaining such load over the grip range including the strength of the blind head.
Therefore it is an object of the present invention to provide a unique blind type fastener including a pin member, a main sleeve and an expander sleeve adapted to form a blind head structure interlocking the expander sleeve into the main sleeve and onto the pin member and a front lock retaining the pin and main sleeve at the accessible side whereby a high clamp load attained on workpieces by the fastener is substantially retained by the locking structure whereby tensile forces resulting from high lap shear loads, which could loosen the fastened joint, are resisted.
Thus it is another object of the present invention to provide a unique blind type fastener including a pin member, a main sleeve and an expander sleeve with a first tulip shaped blind head being formed by movement of the expander sleeve into the main sleeve and a second tulip shaped blind head being formed by subsequent movement of the pin head into the opposite end of the expander sleeve resulting in a high strength blind head and high clamp load with the sleeves and pin member being substantially retained or locked together at the blind side whereby tensile forces resulting from high lap shear loads, which could loosen the fastened joint, are resisted.
It is still another object of the present invention to provide a unique blind type fastener in which a high strength blind head is formed and the final clamp load is secured from reduction due to tensile loads resulting from high lap shear loads.